Positive Displacement Pump With Shaft-Mounted Sleeve

ABSTRACT

A rotary positive displacement pump comprising a pair of forwardly-positioned sealing arrangements and a pair of forwardly-positioned sleeves are received within a cavity providing for easy maintenance of the pump when the seals need to serviced. Each of the sealing arrangement includes a dynamic seal and a static seal. The dynamic seal is positioned forward of the static seal and abuts a corresponding sleeve and hub.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND

The present disclosure relates to rotary positive displacement pumps.More particularly, this disclosure relates to sealing arrangements thatcan be used to seal the rotary components of a positive displacementpump.

Pumps, such as rotary positive displacement pumps, can be used totransport pumped products (e.g., fluids) through a system. In a rotarypositive displacement pump, two or more counter-rotating lobes aredisposed in a cavity typically defined by a pump body and an associatedcover. This cavity has an inlet on one side of the rotary lobes throughwhich the pumped product is initially received and an outlet on theother side of the rotary lobes through which the pumped product isforced out of the pump. A gear case, which typically supports the pumpbody, has shafts that attach to the rotors. When the gear case drivesthe rotation of these shafts, the attached rotors rotate, therebycausing the pumping action.

Although positive displacement pumps of the type described above haveexisted for some period of time, the continued maintenance and serviceof these pumps has presented unique challenges. Various components,including the internal seals, may need periodic replacement. The processfor replacing these components can be both time-consuming and difficult,and can require material handling equipment to perform. With thousandsof pumps of this kind in service throughout the world, there is acontinued need for improvements.

SUMMARY

A rotary positive displacement pump provides an improved sealingarrangement between the pump body and the gear case including aninterposed sleeve with a static seal between a shaft of the gear caseand the sleeve and with a dynamic seal between the sleeve and a forwardpart of the hub. The seal and wear components are easily accessible forservice, replacement, or other maintenance with minimal pumpdisassembly. For example, the pump body does not need to be removed fromthe gear case and the inlet and outlet do not need to be disconnectedfrom attached lines. Accordingly, the forward positioning of the sealingand the wearing component (i.e., the sleeve) allow for significantlyreduced pump downtimes related to cleaning and maintenance operations.

According to one aspect, the rotary positive displacement pump includesa gear case having a pair of shafts extending away from the gear case ata forward end. A pump body is supported by the gear case on the forwardend of the gear case. The pump body has a cover attached thereto, whichtogether define a cavity between the pump body and the cover. The cavityhas an inlet and an outlet and the pump body has a pair of hubs thatextend into the cavity in which each of the pair of hubs has anaxially-extending opening. A corresponding one of the pair of shaftsfrom the gear case is received through each one of the axially-extendingopenings of the hubs. A pair of rotors each have a central portionreceived on one of the pair of shafts inside the axially-extendingopening of the hub. Each rotor has wings attached to the central portionof the rotor in which the wings of the rotor are disposed radiallyoutward of the hub when the central portion of the rotor is attached tothe shaft. The rotors on the pair of shafts are rotatable in oppositedirections to pump a pumped product through the pump body from the inletto the outlet. Notably, the rotary positive displacement pump alsoincludes a pair of sleeves each having a respective sealing arrangement.Each of the pair of sleeves have a wall extending circumferentiallyalong and about a longitudinal axis to define a central opening therein.The central opening of each of the pair of sleeves receives acorresponding one of the pair of shafts such that each of the pair ofsleeves is configured to rotate in concert with the corresponding one ofthe pair of shafts. The pair of sealing arrangements inhibit escape ofthe pumped product from the cavity to a surrounding atmosphere and eachof the pair of sealing arrangements have a dynamic seal and a staticseal. The dynamic seal is positioned forwardly in the axially-extendingopening of the hub and against a radially outward-facing surface of thesleeve. The static seal is positioned within the central opening of thesleeve and abuts a radially inward-facing surface of the sleeve and theshaft.

These and still other advantages of the disclosure will be apparent fromthe detailed description and drawings. What follows is merely adescription of some preferred embodiments of the present disclosure. Toassess the full scope of the disclosure, the claims should be looked toas these preferred embodiments are not intended to be the onlyembodiments within the scope of the claims.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be better understood and features, aspects andadvantages other than those set forth above will become apparent whenconsideration is given to the following detailed description thereof.Such detailed description makes reference to the following drawings.

FIG. 1 is a perspective view of a rotary positive displacement pump.

FIG. 2 is an exploded view of the rotary positive displacement pump ofFIG. 1.

FIG. 3A is a front view of the rotary positive displacement pump of FIG.1.

FIG. 3B is a cross-sectional side view of the rotary positivedisplacement pump taken along line 3B-3B of FIG. 3A.

FIG. 4 is a detailed view of a sealing arrangement in the rotarypositive displacement pump of FIG. 3B, taken along lines 4-4 in FIG. 3B.

Corresponding reference characters indicate corresponding partsthroughout the several views. Although the drawings representembodiments of the present disclosure, the drawings are not necessarilyto scale and certain features may be exaggerated in order to betterillustrate and explain the embodiments of the present disclosure.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to a number ofillustrative embodiments shown in the attached drawings and specificlanguage will be used to describe the same.

FIGS. 1, 2, 3A, 3B, and 4 illustrate an embodiment of a rotary positivedisplacement pump 10 according to the disclosure. The pump 10 can be acircumferential piston pump, for example, which is a type of rotarypositive displacement pump. The pump 10 can be used during theproduction of toothpaste, cosmetics, chocolate, candies, pet food, andother viscous materials to transport slurries or materials throughout aproduction facility, for example. As best seen in FIGS. 1 and 3A,materials can be inputted into the pump 10 through an inlet line 12,through the pump 10, and into the outlet line 14 at a higher pressure orvelocity. Inlet and outlet are relative terms, however, based on thedirection in which the pump 10 is being run, and the inlet and outletcould be reversed.

Generally speaking, the pump 10 includes a gear case 16 with a forwardend that supports a pump body 18 on a rearward end of the pump body 18.The gear case 16 and the pump body 18 may be connected to one another ina number of ways, including using bolts or other fasteners. In someembodiments, the fasteners are readily removable to allow access to theforward end of the gear case 16 or to allow for easy removal of the pumpbody 18 for service. Portions of the gear case 16 including shafts andother rotary components extend into the pump body 18 through the rearside of the pump body 18.

A mounting base 20 is attached to the gear case 16, and can be used tocouple the pump 10 to the floor or other mounting surface. Althoughshown in an upright orientation, the mounting base 20 allows the gearcase 16 to be mounted in a variety of orientations, including bottom,top, and side mounting arrangements. The mounting base 20 can beprovided with a number of holes 22 which can receive fasteners or otheranchoring equipment to secure the pump 10 in a desired location andorientation.

With particular reference now to FIGS. 2, 3B, and 4, the driving andsealing components of the pump 10 are illustrated. The gear case 16 isadapted to translate a single input rotary torque into a pair ofcounter-rotational output rotary torques. In pump 10, the gear case 16is configured or adapted to receive an input rotary torque from a keyedinput shaft 24 at the rear end of the gear case 16. The keyed inputshaft 24 can be coupled to a motor (not shown) using a coupling (notshown) that imparts the input torque onto the input shaft 24. The torquegenerated by the motor and translated to the input shaft 24 is thentranslated and divided within the gear case 16 into a pair of counterrotational output shafts 26 that are positioned on the front end of thegear case 16. The pair of output shafts 26 can be positioned parallel toone another, and can also be positioned parallel to the input shaft 24.Because the translation and division of such torques and rotary motionis known within the gear case art, a greater description of the interiorof the gear case 16 is unnecessary. Moreover, it will be appreciatedthat other styles of gear cases might be used with the improvementsdescribed herein.

The pump body 18 includes a cover 28. Together with the pump body 18,the cover 28 defines a cavity 30 that can receive rotary and sealingcomponents therein. As best illustrated in FIG. 3B, the cavity 30 can beformed between the pump body 18 and the cover 28 and can be configuredto receive and handle viscous materials therein. The cover 28 can beremovably coupled to the pump body 18 using fasteners, for example. Insome embodiments, the cover 28 and pump body 18 are each supported bystuds or connectors that are fastened or otherwise anchored to the gearcase 16. A first set of studs 31 can extend forwardly away from the gearcase 16 into a set of holes formed in the rear end of pump body 18,while a second set of fasteners (not shown) such as a threaded stud witha handle to accommodate turning can be received at openings 33 thatextend into the front end of the pump body 18 through the holes 34 ofthe cover 28. Nuts or other fastening handles can be threaded onto thestuds to secure the cover 28, pump body 18, and gear case 16 to oneanother and different fastening or connecting arrangements might beused. For example, the fastening handles shown and described in U.S.Pat. No. 9,062,676, entitled “Positive Displacement Pump with ImprovedSealing Arrangement and Related Method of Making” can be used to securethe cover 28 to the pump body 18. The contents of the aforementionedpatent are incorporated by reference in their entirety.

As explained above and as depicted in FIGS. 2, 3B, and 4, the cavity 30can house rotary and sealing components of the pump 10. A pair ofintermeshed rotors 36 are coupled to one of each of the output shafts26. The rotors 36 are removably coupled to the output shafts 26, whichdrive rotation of the rotors 36 to produce pumping action. As describedin the previously-mentioned U.S. Pat. No. 9,062,676, the rotors 36 canhave wings 38 that are angularly offset from one another by 90 degrees,such that they intermesh with one another.

The rotors 36 are coupled to the output shafts 26 by telescopicallyinserting a splined opening 40 of a central portion 42 of the rotor 36onto an inversely shaped body of the output shaft 26. In the formillustrated, with the profile of the splined opening 40 having a similarcross-sectional profile to the respective shaft 26 onto which the rotor36 is received, the rotor 36 is precisely angularly driven by therotation of the shaft 26 via the splined engagement. Of course, othertypes of engagement may also be used such as, for example (but notlimited to), keyed engagement. Fasteners 44 are coupled to a threaded,forward end 46 of the output shaft 26 to secure the rotor 36 to theoutput shaft 26, which restricts relative axial movement therebetween.

The central portion 42 of the rotors 36 is received within one of twohubs 48 formed in the pump body 18. The hubs 48 are generallycylindrically tubular shaped and protrude from the rear or base wall ofthe pump body 18. Each of the pair of hubs 48 have a radially inwardfacing surface 50 and a radially outward facing surface 52. As shown inFIG. 2, the radially outward facing surface 52 may extend around only aportion of the total circumference of the hub 48, because a cutoutregion is needed to accommodate the passage of the wings 38 of therotors 36, which extend radially outward from the hub 48. Each of thepair of hubs 48 has an axially-extending opening 54 in which one of thepair of output shafts 26 of the gear case 16 and one of the pair ofrotors 36 is received and secured. Specifically, the central portion 42of the rotors 36 is received within the axially-extending opening 54 ofthe hub 48.

Notably, a pair of sleeves 56 are also received within the cavity 30.Specifically, one of each of the pair of sleeves 56 is received in theaxially-extending opening 54 of each hub 48 and is interposed betweenthe shaft 26 (with which the sleeve 48 is rotationally coupled) and theradially inward facing wall of the hub 48.

Each sleeve 56 includes a wall 58 extending circumferentially along andabout a longitudinal axis X-X to define a central opening 60 therein.The forward end of each of the pair of sleeves can have a boss 61protruding outwardly therefrom. The generally cylindrical outer shape ofthe wall 58 can taper outward to form the boss 61, which can be used tosupport sealing arrangements, as explained in more detail below.

With specific reference to FIGS. 3B and 4, the sleeve 56 profile andpositioning within the greater assembly of pump 10 is shown inadditional detail. The central opening 60 of the sleeve 56 is receivedon the shaft 26 and the sleeve 56 is secured on the shaft 26 by theattachment of the rotor 36 to the shaft 25 (with the central portion 42of the rotor 36 being received between the forward end of the shaft 26and the sleeve 26 such that the sleeve 26 is radially positioned betweenthe central portion 42 of the rotor 36 and the radially inward facingwall of the hub 48). To accommodate and closely fit these components ofdifferent sizes, the central opening 60 of the sleeve 56 can have avarying shape. For example, the central opening 60 of the sleeve 56 canbe defined by a rear section 62 and a forward section 64. The rearsection 62 can be defined by a radius smaller than the radius definingthe forward section 64, since the rear section 62 surrounds a portion ofthe shaft 26, while the forward section receives the central portion 42of the rotor 36, which has a greater diameter than some portions of theshaft 26.

The sleeve 56, along with a sealing arrangement between the sleeve 56,the shaft 26, and the hub 48, help inhibit the escape of the pumpedproduct from the cavity 30 to the surrounding atmosphere. Sealingarrangements are located about each shaft 26 and rotor 36, and typicallyinclude at least one dynamic seal 68 and at least one static seal 70. Asshown in FIG. 4, the dynamic seal 68 is positioned forwardly in theaxially-extending opening 54 of the hub 48 and against a radiallyoutward facing surface 72 of the sleeve 56. In the context used,“forwardly” in the axially-extending opening 54 means “within” theaxially-extending opening 54 of the hub 48. The dynamic seal 68 could bepositioned just forward of the rear surface 74 of the axially-extendingopening 54, within the forward-most half of the axially-extendingopening 54 of the hub 48, or within a forward-most quadrant of theaxially-extending opening 54 of the hub 48, for example.

In some embodiments, the dynamic seal 68 is positioned against aradially-outward facing surface of the boss 61. As shown, the boss 61can be received entirely within the axially-extending opening 54 of thehub 48, and rearward from the wings 38 of the rotor 36. In someembodiments, the wings 38 of the rotor 36 can restrict axial movement ofthe sleeve 56 relative to the shaft during pump 10 operation.

Typically, a circumferentially-extending groove 75 is formed within theradially inward facing surface of the axially-extending opening 54 toreceive the dynamic seal 68 therein. The circumferentially-extendinggroove 75 can be formed into the axially-extending opening 54 in anumber of ways, including casting, milling, or other suitable machiningoperations. By placing the dynamic seal 68 (here, an o-ring) in thegroove 75 of the hub, the o-ring can remain fix relative to the hub 48and any wear occurs between the o-ring and the sleeve 56, which sleeve56 can be sacrificial and easily replaced relative to the other largercomponents (i.e., the pump body 18) of the pump 10.

Due to the limited thickness of material present in this portion of thehub 48 and the difficulty in adequately sealing the axially-extendingopening 54 from fluid entry, dynamic seals 68 and grooves 75 havetraditionally been positioned outside of the axially-extending opening54 of the hub 48, where more robust seals can be easily positioned. Thelimited cross-section of the axially-extending opening 54 of the hub 48presents machining challenges that have prevented others from placing adynamic seal 68, such as that illustrated, forward in theaxially-extending opening 54 of the hub 48. With the elongated sleeve 56design incorporated into the pump 10, however, the forward location ofthe groove 75 and the dynamic seal 68 together provide sealing betweenthe sleeve 56 and the hub 48. In the orientation shown, the sleeve 56(which as mentioned above can be considered a sacrificial wearcomponent) and dynamic seal 68 can be removed and replaced easily,without removing the pump body 18 from the gear case 16, saving time andlabor during cleaning and servicing of the pump 10, as the entire pumpbody 18 does not need to be removed to access rearwardly positionedseals.

Accordingly, the dynamic seal 68 provides a seal between the radiallyoutward facing surface of the sleeve 56 and the radially inward facingsurface of the hub 48, with the sleeve 56 (along with the shaft 26 andthe rotor 36) rotating relative to the hub 48 as pump 10 operates. Inorder to drive the sleeve 56 in concert with the shaft 26, one or moreaxially-extending notches 76 may be formed in a rear surface 78 of thesleeve 56. One of the axially-extending notches 76 receive a pin 80extending radially outward from the shaft 26. Although one notch issufficient, it is contemplated that multiple notches can be present tomake it easier to locate the pin in one of the notches during placementof the sleeve on the shaft (that is, to find an angular position inwhich the pin is accepted into one of the slots). For example, in theillustrated embodiment, four notches offset by 90 degrees from oneanother are present. In some embodiments, the pin 80 is pressed into theshaft 26, and can have a generally cylindrical shape. The notch 76 canbe defined by an arc length at least two times larger than the width(e.g., the diameter) of the pin 80, which can ease the installation ofthe sleeve 56 onto the shaft 26 during assembly. With this construction,when the shaft 26 rotates, the pin 80 coupled to the shaft 26 rotates aswell, and engages the notch 76, thereby causing the sleeve 56 to rotatein concert with the shaft 26. Axial movement of the sleeve 56 isrestricted by the wings 38 of the rotors 36 and a shoulder 82 formed inthe shaft 26 which is generally achieved by capturing the sleeve 56 onthe shaft 26 by securing the rotor 36 on the shaft 26.

As mentioned above, there is also a static seal 70 provided between thesleeve 56 and the shaft 26. The static seal 70 is positioned within thecentral opening of the sleeve 56 and abuts a radially inward-facingsurface 84 of the sleeve 56 and abutting the shaft 26. In someembodiments, the static seal 70 abuts the radially inward-facing surface84 of the rear section 62 of the sleeve 56. One or more grooves 86, 88may be present within the shaft 26 and can be used to position thestatic seal 70 (typically an o-ring) between the sleeve 56 and the shaft26. Because the sleeve 56 and shaft 26 rotate together during pumpoperation, virtually no relative movement between the components occurs,thus enabling the use of a static seal 70.

As illustrated, the static seal 70 can be an O-ring. The O-ring can beformed of silicone, Kalrez® (or other perfluoroelastomers), nitrilerubber, Viton® (or other fluoroelastomers), ethylene propylene rubber,or other suitable materials for establishing static seals in sanitaryenvironments. The dynamic seal 68 can be a reciprocating or rotary sealformed of polymeric or composite materials, for example.

Using the above-described sealing arrangement within the pump 10, aforward sealed rotary positive displacement pump can be effectivelyoperated and maintained. The wear components, including the dynamic seal68, static seal 70, and sleeve 58 effectively seal the pump cavity 30and restrict the escape of pumped product, fluid, or contaminants fromthe cavity 30 into the surrounding atmosphere or environment.

The forward positioning of the wear components within the body 18 makeseach of the sleeve 56, dynamic seal 68, and static seal 70 readilyaccessible and replaceable, in case repair or maintenance is required ordesired. To access the components within the pump 10, the cover 28 canfirst be removed from the pump body 18. Next, a wrench or other tool canbe used to remove the fasteners 44 from the forward end 46 of the shafts26. The rotor 36 can then be pulled axially away from the shaft 26,thereby removing the splined opening 40 of the central portion 42 of therotor 36 from the output shaft 26 and exposing the forward section 64 ofthe sleeve 56 to the external environment. At this point, the sleeve 56can be dislodged or displaced relative to the hub 48 by inserting ascrewdriver into an available space of one of the notches 76 andslightly twisting the screwdriver to dislodge the sleeve 56 from theshaft 26. The sleeve can be further removed by inserting a pick into aforward grove on the inside of the sleeve 56. In some embodiments, amaintenance worker can remove the sleeve 56 by hand once the forward end64 protrudes outward from the hub 48. The screwdriver can be used onceagain to remove the dynamic seal 68 from the groove 75, should it benecessary. With the sleeve 56 and dynamic seal 68 removed, a user canalso access, clean, or replace the static seal 70, through thecircumferential clearance between the shaft 26 and the axially-extendingopening 54 of the hub 48.

To reassemble the pump, the dynamic seal 68 can be initially fitted intothe groove 75 of the hub 48. Next, the sleeve 56 is passed into theaxially-extending opening 54 of the hub, until the rear section 62 ofthe sleeve engages the shoulder 82. In order to produce engagementbetween the rear section 62 of the sleeve and the shoulder 82, the pin80 should be radially positioned within the axially-extending notch 76formed in the sleeve 56. Because the axially-extending notch 76 isdefined by an arc length at least twice the width of the pin 80, thenecessary angular positioning precision of the sleeve 56 relative to theshaft 26 is relaxed, which helps decrease installation time. Once thesleeve 56 is properly positioned within the hub 48, the rotor 36 can bereplaced, so that the splined opening 40 reengages the output shaft 26.The fasteners 44 can be recoupled to the forward end of the shaft 46,which locks the axial positioning of the rotor 36 and the sleeve 56.Then, the cover 28 can be recoupled to the pump body 18, so that thecavity 30 is restored and pump operation can resume.

Finally, it should be appreciated that because the pump 10 has a pair ofshafts 26, rotors 36, and hubs 48, that there can be a correspondingsleeve 56 and corresponding sealing arrangement (i.e., static anddynamic seals) at each one of the shaft-rotor-hub assemblies. Althoughthis should be apparent from the figures and description above, it isnonetheless expressly stated to make clear that there are in fact a pairof sleeves (each with a respective sealing arrangement) at each of theshafts, rotors and hub sets.

It should be appreciated that various other modifications and variationsto the preferred embodiments can be made within the spirit and scope ofthe disclosure. Therefore, the disclosure should not be limited to thedescribed embodiments. To ascertain the full scope of the disclosure,the following claims should be referenced.

What is claimed is:
 1. A rotary positive displacement pump for pumping apumped product, the rotary positive displacement pump comprising: a gearcase including a pair of shafts extending therefrom at a forward end; apump body supported by the gear case on the forward end of the gearcase, the pump body having a cover attached thereto so as to define acavity between the pump body and the cover in which the cavity has aninlet and an outlet, the pump body having a pair of hubs that extendinto the cavity in which each of the pair of hubs has anaxially-extending opening through each one of which a corresponding oneof the pair of shafts from the gear case is received; a pair of rotors,each of the pair of rotors having a central portion received on one ofthe pair of shafts inside the axially-extending opening of the hub, eachrotor having wings attached to the central portion of the rotor in whichthe wings of the rotor are disposed radially outward of the hub when thecentral portion of the rotor is attached to the shaft, the rotors on thepair of shafts being rotatable in opposite directions to pump a pumpedproduct through the pump body from the inlet through the cavity to theoutlet; a pair of sleeves, each of the pair of sleeves having a wallextending circumferentially along and about a longitudinal axis todefine a central opening therein, the central opening of each of thepair of sleeves receiving a corresponding one of the pair of shafts, andeach of the pair of sleeves configured to rotate in concert with thecorresponding one of the pair of shafts; and a pair of sealingarrangements corresponding to the respective pairs of shafts and sleevesto inhibit the escape of the pumped product from the cavity to asurrounding atmosphere, each of the pair of sealing arrangements havinga dynamic seal and a static seal in which the dynamic seal is positionedforwardly in the axially-extending opening of the hub and against aradially outward-facing surface of the sleeve and in which the staticseal is positioned within the central opening of the sleeve and abuts aradially inward-facing surface of the sleeve and the shaft.
 2. Therotary positive displacement pump of claim 1, wherein a rear surface ofthe sleeve is positioned adjacent a radially outward-extending shoulderof the shaft and a forward surface of the sleeve is positioned adjacentthe wings of the rotor.
 3. The rotary positive displacement pump ofclaim 2, wherein an axially-extending notch is formed in the rearsurface of the sleeve.
 4. The rotary positive displacement pump of claim3, wherein a pin extends radially outwardly from the shaft and into thenotch in the sleeve.
 5. The rotary positive displacement pump of claim4, wherein the pin is pressed into the shaft.
 6. The rotary positivedisplacement pump of claim 4, wherein the notch of the sleeve has an arclength that is at least two times larger than a width of the pin.
 7. Therotary positive displacement pump of claim 6, wherein the pin has acylindrical shape and the width of the pin corresponds to a diameter ofthe pin.
 8. The rotary positive displacement pump of claim 1, whereinthe static seal is an o-ring.
 9. The rotary positive displacement pumpof claim 8, wherein the o-ring is formed from a material chosen from thegroup consisting of silicone, perfluoroelastomer, nitrile rubber,fluoroelastomer, and ethylene propylene rubber.
 10. The rotary positivedisplacement pump of claim 1, wherein a groove extends circumferentiallyabout the axially-extending opening to receive the dynamic seal therein.11. The rotary positive displacement pump of claim 1, wherein thedynamic seal is positioned within a forward-most half of theaxially-extending opening of the hub.
 12. The rotary positivedisplacement pump of claim 11, wherein the dynamic seal is positionedwithin a forward-most quarter of the axially-extending opening of thehub.
 13. The rotary positive displacement pump of claim 1, wherein thecentral opening of each of the pair of sleeves includes a rear sectiondefined by a first radius and a forward section defined by a secondradius in which the second radius is larger than the first radius. 14.The rotary positive displacement pump of claim 13, wherein each forwardsection of the pair of sleeves receives the central portion of one ofthe pair of rotors therein.
 15. The rotary positive displacement pump ofclaim 13, wherein the static seal abuts a radially inward-facing surfaceof the rear section of the sleeve.
 16. The rotary positive displacementpump of claim 15, wherein the static seal is received within a grooveformed in the shaft.
 17. The rotary positive displacement pump of claim1, wherein a forward end of each of the pairs of sleeves includes a bossprotruding therefrom.
 18. The rotary positive displacement pump of claim17, wherein the dynamic seal is positioned against a radially-outwardfacing surface of the boss.
 19. The rotary positive displacement pump ofclaim 17, wherein the boss is received entirely within theaxially-extending opening of the hub.
 20. The rotary positivedisplacement pump of claim 1, wherein each of the pair of sealingarrangements includes a second static seal positioned within the centralopening of the sleeve, abutting the radially inward-facing surface ofthe sleeve and the shaft.